1. Field of the Invention
This invention relates to a digital data processing system of the type having a plurality of memory units connected to a processing unit by means of a common bus. More particularly it relates to a multiple memory system in which the data retrieval operations of the memory units are overlapped so that retrieval in one memory unit can begin before a previous retrieval in another unit has been completed.
2. Prior Art
The present invention is an improvement on data processing systems of the type described in U.S. Pat. No. 3,376,554. One of these systems will typically have several memory units that store data received from one or more processors and, conversely, transmit stored data to the processors. The memories are connected to each processor unit by a common bus arrangement. When a processor it so perform a memory operation it transmits an address signal onto an address section of the bus and the memory containing the prescribed address responds by connecting itself to the bus and signalling the connection to the processor unit. Thereupon the processor unit transmits data over a data section of the bus if the operation is a write (store) operation or the memory unit transmits stored information if it is a read (retrieve) operation.
Normally, before a memory operation can be initiated, the previous memory operation must be completed, even though the two operations involve different memory units. This presents no problem with write operations because the memory units can be uniformly very fast in accepting data for storage. That is, even a slowly operating memory unit can temporarily store incoming data in a high speed register and then transfer the data into the designated memory address in subsequent internal operation. Thus the speed of this internal operation does not affect the time required for data transfer to the memory unit.
On the other hand, during a retrieval operation, data cannot be transmitted from a memory unit to the processor unit until the memory unit has retrieved the data from the designated address therein. Accordingly, the internal operating time of the memory does affect the overall time required for data retrieval. For a very fast memory unit, for example those using flip-flop registers as storage media, the read operation is very fast. However, when a slower memory is involved, the retrieval of stored data takes appreciable time. The required time may be further extended by the present day trend toward locating various units of the data processing system at significant distances from each other. The memory units may be located so far from associated processor units that the propagation time on the cable connecting the two units materially limits the system operating speed.